The World Boxing Super Series cruiserweight tournament, already stacked with a who's who of the weight class, snared its biggest fish yet with the addition of world titleholder Oleksandr Usyk, who officially entered the field Saturday.

Many regard Usyk as the No. 1 fighter in the world in the 200-pound division, but he will have a chance to prove it if he can win the eight-man, single-elimination tournament that also includes three other world titleholders, Russia's Murat Gassiev (24-0, 17 KOs), Latvia's Mairis Briedis (22-0, 18 KOs) and Cuban defector Yunier Dorticos (21-0, 20 KOs), and two highly regarded former titleholders, Poland's Krzysztof Wlodarczyk (53-3-1, 37 KOs) and Germany's Marco Huck (40-4-1, 27 KOs).

Oleksandr Usyk is the fourth cruiserweight titleholder to join the World Boxing Super Series tournament. Harry How/Getty Images

The two remaining slots are expected to be filled by Poland's Krzysztof Glowacki (27-1, 17 KOs), another former world titleholder, and Russian knockout artist Dmitry Kudryashov (21-1, 21 KOs).

Usyk (12-0, 10 KOs), a 30-year-old southpaw who won the 2012 Olympic heavyweight gold medal for Ukraine, has said since turning pro that he wanted to unify the cruiserweight division and then move up to heavyweight. He will have that chance to unify in the Super Series, the key reason why he said he joined, as expected.

"I feel happy and inspired with the idea of such a tournament," Usyk said. "I've been dreaming of putting together all the champs to see who is the strongest and becoming the undisputed king of the division. I look forward to gaining three more world championship belts to my collection as well as the Muhammad Ali Trophy."

n March, Comosa AG announced plans for two tournaments in which 16 fighters would divvy up $50 million in prize money, with the winner of each tournament also receiving the Muhammad Ali Trophy. Comosa AG enlisted promoters Richard Schaefer and Kalle Sauerland to promote the cards and decided that the first two tournaments would take place in the cruiserweight and super middleweight divisions.

The tournaments are supposed to kick off in September with quarterfinal matches in each weight class. The semifinals are scheduled for early 2018 and the finals in May 2018. The draw for each tournament will take place on July 8 in Monte Carlo, Monaco, with the top four seeds, which are to be determined, in each field selecting their quarterfinal opponent from the remaining unseeded fighters.

In September, Usyk set the cruiserweight division record for fewest fights needed to win a world title when he broke Evander Holyfield's mark of 12 by outpointing Glowacki to win a belt in his 10th professional bout, doing so on Glowacki's home turf in Poland.

Usyk's first two defenses came in the United States on HBO-televised undercards. He scored a ninth-round knockout of Thabiso Mchunu in December and a lopsided unanimous decision against Michael Hunter Jr. on April 8.

Plans were in the works for his third defense to take place in August in Ukraine. Now that fight will be pushed back to his tournament opener in the fall, though it likely will still take place in Ukraine.

"This is a historic moment," said Roberto Dalmiglio of Comosa AG. "Nobody has ever assembled as many top fighters as the World Boxing Super Series and put them in an elimination tournament to determine the undisputed No. 1. Whoever emerges from a competition that features the four reigning champions, two former champions as well as two rising challengers to be announced shortly will be universally recognized as the greatest cruiserweight out there."

Three of the eight slots in the super middleweight tournament field have been filled by England's George Groves (26-3, 19 KOs), who holds one of the 168-pound world titles, Sweden's Erik Skoglund (26-0, 12 KOs) and England's Jamie Cox (22-0, 12 KOs).